44O DEVELOPMENT OF ELASMOBRANCH FISHES. 



obtained by an examination of the nerves ; the third column is 

 for the visceral arches (which lead to the same results as, but are 

 more convenient for our table than, the visceral clefts), and the 

 fourth column is for the head-cavities. It may be noticed that 

 from the second segment backwards the three sets of organs 

 lead to the same results. The head-cavities indicate one seg- 

 ment in front of the mouth, and now that the ophthalmic branch 

 of the fifth has been dethroned from its position as a separate 

 nerve, the eye-nerves, or one of them, may probably be regarded 

 as belonging to this segment. If the suggestion made above 

 (p. 431), that the walls of the first cavity become the eye- 

 muscles, be correct, the eye-nerves would perhaps after all be 

 the most suitable nerves to regard as belonging to the segment 

 of the first head-cavity. 



EXPLANATION OF PLATES 15, 16, 17. 



PLATE 15. (THE HEAD DURING STAGES G K.) 

 COMPLETE LIST OF REFERENCE LETTERS. 



i aa, iaa, etc. * ist, 2d, etc. aortic arch. acv. Anterior cardinal vein. al. Ali- 

 mentary canal, ao. Aorta. an. Thickening of epiblast to form the auditory pit. 

 aun. Auditory nerve, aup. Auditory pit. auv. Auditory vesicle, b. Wall of 

 brain, bb. Base of brain. cb. Cerebellum, cer. Cerebrum. Ch. Choroid slit. 

 ch. Notochord. com. Commissure connecting roots of vagus nerve, i, 2, 3 etc. 

 eg. External gills, ep. External epiblast. fb. Fore-brain, gl. Glossopharyngeal 

 nerve, h b. Hind-brain, ht. Heart, hy. Hyaloid membrane. In. Infundibulum. 

 /. Lens. M. Mouth involution, m. Mesoblast at the base of the brain, m b. Mid- 

 brain, mn. v. Mandibular branch of fifth, ol. Olfactory pit. op. Eye. opn. Optic 

 nerve, opv. Optic vesicle, opth v. Ophthalmic branch of fifth, p. Posterior root 

 of spinal nerve, pn. Pineal gland. 1,2 etc. pp. First, second, etc. section of body- 

 cavity in the head. pt. Pituitary body. so. Somatopleure. sp. Splanchnopleure. 

 spc. Spinal cord. Th. Thyroid body. v. Blood-vessel, iv. v. Fourth ventricle, 

 v. Fifth nerve. V ' c. Visceral cleft. Vg. Vagus, vii. Seventh or facial nerve. 



Fig. i. Head of a Pristiurus embryo of stage K viewed as a transparent object. 



The points which deserve special attention are: (i) The sections of the body- 

 cavity in the head (pp) : the first or premandibular section being situated close to the 

 eye, the second in the mandibular arch. Above this one the fifth nerve bifurcates. 

 The third at the summit of the hyoid arch. 



The cranial nerves and the general appearance of the brain are well shewn in the 

 figure. 



